Cellulose-precursor carbon fibers generally present a porous structure made up of highly disorganized turbostratic carbon, said structure also being highly disoriented relative to the axial direction of the fibers and their pore lattices.
Those characteristics confer low thermoconductivity on the carbon fibers, thereby making them particularly suitable for forming thermal protection coatings, such as ablative coatings for combustion chambers and thruster nozzles.
Other applications have been envisaged for cellulose-precursor carbon fiber fabric, in particular for making heating resistors, making battery electrodes, or catalyst supports, or forming activated fabric used as absorbent material.
Methods of obtaining fabric made of cellulose-precursor carbon fibers are known. Reference can be made in particular to U.S. Pat. Nos. 3,053,775, 3,107,152, 3,305,315, and 3,663,173.
A commonly used method consists in performing direct carbonization on a cellulose fiber fabric, in particular a viscose fabric. The fabric is put into the form of a hank that is several hundreds of meters (m) long. It is precarbonized up to a temperature of about 400° C. Precarbonization is performed in a container, preferably under an inert atmosphere, e.g. while being swept with nitrogen. The effluents coming from the decomposition of the cellulose are sucked away and burned off in a flare.
Temperature rises very slowly so as to comply with the decomposition kinetics of cellulose, so as to obtain a satisfactory yield of carbon, and so as to ensure that the decomposition reaction, which is exothermic, does not run away, since such a runaway could destroy the mechanical properties of the resulting carbon fibers. By way of example, for a 100 m long hank, precarbonization can last for as much as 15 days, which is extremely long.
The precarbonization stage is followed by heat treatment at a temperature of about 1200° C. for about 1 minute (min) to 2 min. Final treatment at high temperature, e.g. as high as 2800° C., can be performed to increase the conductivity of the carbon and close its pores.
A method and an installation for obtaining carbon fiber fabric by continuously carbonizing a cellulose fiber fabric with heat treatment lasting for a much shorter duration is described in Russian patents Nos. 2 005 829, 2 045 472, and 2 047 674.
The precursor fabric, e.g. of engineering viscose fibers, is impregnated by an organosilicon compound having the effect of conserving good mechanical properties for the resulting carbon fiber fabric. The organosilicon compound is selected from compounds in the group: polydimethylphenylallylsilanes, polysiloxanes, polymethylsiloxanes, polysilazanes, and polyalumunio-organosiloxanes.
The impregnated fabric is subjected to continuous heat treatment in air at a temperature lying in the range 100° C. to 300° C., and more particularly in the range 100° C. to 150° C. so as to relax the stresses which exist in the cellulose fibers and so as to eliminate the water adsorbed by the fibers.
Carbonization is then performed on the fabric passing continuously through an enclosure under an inert atmosphere, with the temperature being raised progressively up to 300° C. to 600° C. High temperature treatment up to a maximum of 2800° C. under an inert atmosphere is then performed.
During carbonization, the gas effluents of cellulose pyrolysis are sucked up and burned off in a flare, with the suction means being located in the enclosure where most cellulose degradation takes place.
That method makes it possible to obtain satisfactory mechanical properties for the carbon fibers, but it leads to the resulting fabric being deformed, e.g. by disorganizing its weave and by warp shrinkage.
Such deformation is not acceptable, in particular when the fabric is to be used for making preforms for composite material parts, since the deformation leads to fibers being distributed in non-uniform manner within the preform, and that affects the behavior of composite material parts reinforced by such fabric.